Maximal value of the Boost Multiprecision type 'float128' gives me a compilation error with GCC in C++20 mode

Question

This simple code can't be compiled with the -std=c++20 option:

#include <limits>
#include <boost/multiprecision/float128.hpp>

namespace bm = boost::multiprecision;

int main()
{
  auto const m = std::numeric_limits<bm::float128>::max();
}

The compilation command:

g++ -std=c++20 -Wno-unused-but-set-variable test.cpp

The compiler complains about the BOOST_MP_QUAD_MAX constant. Why is that?

• OS: Xubuntu 20.04.4 LTS
• Compiler: g++ (Ubuntu 10.3.0-1ubuntu1~20.04) 10.3.0

Answer 1

Directly from the documentation :

When compiling with gcc, you need to use the flag --std=gnu++11 / 14 / 17 , as the suffix Q is a GNU extension. Compilation fails with the flag --std=c++11 / 14 / 17 unless you also use -fext-numeric-literals .

The same applies for c++20 / gnu++20 , the docs merely haven't yet been updated for that revision.

Answer 2

The documentation states:

When compiling with gcc, you need to use the flag --std=gnu++11/14/17, as the suffix 'Q' is a GNU extension. Compilation fails with the flag --std=c++11/14/17 unless you also use -fext-numeric-literals.

So you need to specify --std=gnu++20 instead of --std=c++20 . The boost documentation is not up-to-date. The flag enables various GNU extensions , one of it being __float128 .

See example on godbolt .

The underlying reason for the compiler error is that without the --std=gnu++17 or --std=gnu++20 flag, _GLIBCXX_USE_FLOAT128 is not defined, meaning BOOST_HAS_FLOAT128 is not defined, meaning __float128 is not recognized as number_kind_floating_point (but as number_kind_unknown ). This means that boost::multiprecision::number cannot be implicitly constructed from a __float128 because is_restricted_conversion<__float128, boost::multiprecision::float128_backend>::value is true because bm::detail::is_lossy_conversion<__float128, boost::multiprecision::float128_backend>::value is true , because __float128 is number_kind_unknown instead of number_kind_floating_point .

Ie in short, without gnu++20 , the boost::multiprecision::float128 type is broken.